EP0499262A2 - Retro-laser Raman à foyers multiples - Google Patents

Retro-laser Raman à foyers multiples Download PDF

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Publication number
EP0499262A2
EP0499262A2 EP92102460A EP92102460A EP0499262A2 EP 0499262 A2 EP0499262 A2 EP 0499262A2 EP 92102460 A EP92102460 A EP 92102460A EP 92102460 A EP92102460 A EP 92102460A EP 0499262 A2 EP0499262 A2 EP 0499262A2
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EP
European Patent Office
Prior art keywords
raman
backward
srs
wave
focus
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Granted
Application number
EP92102460A
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German (de)
English (en)
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EP0499262B1 (fr
EP0499262A3 (en
Inventor
Donald R. Dewhirst
Robert D. Stultz
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Raytheon Co
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Hughes Aircraft Co
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/30—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range using scattering effects, e.g. stimulated Brillouin or Raman effects
    • H01S3/305—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range using scattering effects, e.g. stimulated Brillouin or Raman effects in a gas
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/30—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range using scattering effects, e.g. stimulated Brillouin or Raman effects

Definitions

  • the present invention relates to a laser apparatus and method for producing a high quality, low divergence optical beam having a wavelength shifted from that of an input optical pump beam by a Stokes shift due to stimulated Raman scattering (SRS).
  • SRS stimulated Raman scattering
  • Laser sources for various applications are required to emit high quality, coherent beams at a wavelength which will not damage the human eye.
  • the wavelength of 1.54 microns has been generally established as being "eye-safe".
  • a Nd:YAG laser in widespread use is capable of producing a high quality beam at 1.06 microns, which is outside of the eye-safe range.
  • Raman laser devices utilizing the Stokes shift in a Raman scattering medium can be used to convert laser radiation of one wavelength to a longer wavelength.
  • Methane is a Raman medium having a vibrational Stokes frequency shift of 2916 cm ⁇ 1, enabling conversion of a 1.06 micron beam to a 1.54 micron beam. This conversion may be accomplished as described, for example, in U.S. Patent No. 4,821,272, entitled “SINGLE MIRROR INTEGRAL RAMAN LASER", issued April 11, 1989 to H. Brusselbach et al, and assigned to the same assignee as the present application.
  • Raman scattering of an input optical beam in a suitable medium produces both forward and backward propagating SRS waves, as described in a basic treatise on stimulated Raman scattering found in "Tunable Lasers", by J. C. White, Springer Series Topics in Applied Physics, Vol. 59, Springer, Berlin, Heidelberg, 1987, pp. 115-207.
  • the arrangement disclosed in the Brusselbach patent is known as a "Raman half-resonator", and produces a Raman-shifted output using the forward SRS wave.
  • the beam divergence of this type of laser is on the order of twice the pump beam divergence, which is undesirable for certain applications.
  • the backward SRS wave is retro-reflected back onto the input pump beam. This reduces the number of optical elements that affect output beam alignment in a backward Raman laser.
  • the backward SRS wave does not parametrically couple to anti-Stokes radiation, whereas the forward SRS wave does.
  • This coupling of the SRS wave to anti-Stokes reportedly causes an effective reduction in gain for the lower order modes of the forward SRS wave, and therefore increases forward Raman beam divergence, as discussed, for example, in the publication by Perry et al., "Stimulated Raman Scattering With a Tightly Focused Pump Beam", Optics Letters , Vol. 10, No. 3, 146 (1985).
  • the present invention provides a method and apparatus which improve upon the single focus backward SRS laser by providing at least one additional focus in the Raman medium for generating a backward SRS wave which propagates back into the first focus to act as a seed for SRS.
  • the seed beam has an intensity which is much greater than the seed from spontaneous Raman scattering in the first focus.
  • This seed beam interacts with and is amplified by the pump beam at the first focus to produce an output backward SRS wave with greater conversion efficiency than is attainable with a single focus.
  • the present invention can be operated efficiently at much lower pump input power than a single focus backward SRS laser.
  • the present invention also has the advantages of the single focus backward SRS laser in that sensitivity to optics misalignment is reduced and the output beam has low divergence which is comparable to the pump beam.
  • the apparatus of the present invention comprises:
  • the first and second or subsequent focal points may be located within the same or separate Raman cells.
  • FIG. 1 illustrates a single cell, two focus backward Raman laser apparatus in accordance with a preferred embodiment of the present invention, which is generally designated as 10, and includes a pump laser 12 for generating a coherent optical pump beam 14.
  • the laser 12 will typically be a Q-switched Nd:YAG laser producing the pump beam 14 at a wavelength of 1.06 microns.
  • the present invention may be used with any laser which is known for use with Raman cells and any Raman medium.
  • the laser 12 is disposed such that the pump beam 14 is caused to propagate through a first Raman cell focus 34 and a second Raman cell focus 36 in Raman cell 16.
  • propagation from left to right in the figure will be considered to be a forward direction, with propagation from right to left being a backward direction.
  • the cell 16 is filled with Raman medium 20 which produces the SRS effect.
  • Raman medium 20 which produces the SRS effect.
  • Flat windows 21 and 23 are provided to contain the Raman medium in the cell 16.
  • the preferred medium for producing an eye-safe output beam at 1.54 microns is pressurized methane gas, although deuterium, which has a vibrational Stokes frequency shift of 2991 cm ⁇ 1, is also applicable.
  • the medium 20 is methane gas
  • it will be maintained at a pressure within the range of approximately 600 to 1000 pounds per square inch gage (psig) or 42.2 to 70.3 kilograms per centimeter2 (Kg/cm2), which is selected to promote SRS and suppress stimulated Brillouin scattering (SBS).
  • psig pounds per square inch gage
  • Kg/cm2 42.2 to 70.3 kilograms per centimeter2
  • SBS gain becomes insufficient at pressures lower than 600 psig
  • SBS gain becomes excessive at pressure above 1000 psig.
  • Operation at increased pulse repetition frequency (PRF) may be facilitated by circulating the medium 20 at the focuses 34 and 36 using a gas circulation unit 24. Since substantially all of the SRS takes place at the first focus 34, circulation of the gaseous Raman medium 20 at only the first focus 34 usually will be adequate.
  • PRF pulse repetition frequency
  • a converging lens 26 focuses the pump beam 14 into the cell 16 to provide sufficient intensity to exceed the Raman threshold in the cell 16 and enable SRS to occur.
  • a second lens 28 re-focuses the pump beam 14 within the cell 16.
  • a third lens 32 re-collimates the beam after propagation in the forward direction through the cell 16.
  • the backward SRS wave generated at the focal point 36 in the Raman cell 16 propagates in the backward direction and acts as a seed for backward SRS at the focus 34.
  • the intensity of the backward seed is much greater than the spontaneous Raman scattering at focus 34, and thus the backward SRS wave is amplified at the focus 34 through interaction of the backward seed with the pump beam 14.
  • the Raman medium 20 is methane gas and the wavelength of the pump beam 14 is 1.06 microns
  • the backward SRS wave will have the eye-safe wavelength of 1.54 microns due to the vibrational Stokes shift produced by SRS. Improved operation is facilitated by reducing the distance between the focal points 34 and 36 and thereby the transit time therebetween as much as possible.
  • the backward SRS wave which constitutes the output beam of the apparatus 10, and is designated as 38 in FIG. 1, is extracted by an output means, such as a dichroic beamsplitter 40, which directs the output beam 38 external of the apparatus 10 for use in the desired application.
  • the beamsplitter 40 is a conventional wavelength sensitive optical element designed to transmit the 1.06 micron pump beam 14 therethrough and reflect the 1.54 micron backward SRS beam therefrom as the output beam 38.
  • the backward SRS wave is internally retro-reflected back toward the pump beam 14 in the cell 16, regardless of movement or misalignment of intervening optical elements. This eliminates the optical elements of the Raman cell from affecting the output beam alignment, thereby simplifying the design and reducing the manufacturing cost of the apparatus.
  • the output beam 38 has low divergence, on the order of 1.3 times the divergence of the pump beam 14, compared to a factor of two for the prior art Raman half resonator configuration.
  • the forward SRS wave generated in the cell 16 may be unused and allowed to exit from the apparatus 10 from the cell 16 or may be absorbed by known means (not shown). However, as a preferred alternative, the forward SRS wave may be reflected in the backward direction through the cell 16 to further increase the backward SRS seed beam. This function may be performed by a reflector such as a corner cube 42 disposed downstream of the cell 16 in the forward direction. The preferred position for the corner cube 42 is as close to the adjacent end of the cell 16 as possible, although the performance does not deteriorate substantially if the cube 42 is moved away from the cell 16 by a relatively small distance.
  • the backward SRS polarization is identical to the pump beam polarization, even though the corner cube 42 depolarizes the forward SRS wave. This demonstrates that the Raman conversion occurs almost entirely as backward SRS. Only the Raman seed component polarized like the pump beam 14 has gain. It is within the scope of the invention to replace the corner cube 42 with an alternative reflector, such as a plane mirror (not shown). This results in even higher Raman conversion efficiency since a plane mirror does not de-polarize the feedback beam.
  • a pump beam dump in the form of a dichroic beamsplitter 44 is provided between the cell 16 and the corner cube 42, to prevent the residual or depleted pump beam, here designated as 46, from re-entering the cell 16 and pump laser 12.
  • the beamsplitter 44 operates on the same principle as the beamsplitter 40, and is designed to transmit the 1.54 micron forward SRS beam, and to reflect the 1.06 micron depleted pump beam 46 external of the apparatus 10. If the corner cube 42 is not used, the pump beam dump is not required.
  • An optical isolator generally indicated as 48, may be provided for preventing an SBS wave 50 generated in the cell 16 from propagating backwardly into the pump laser 12.
  • the unit 48 may include a polarizer 52 and a quarter wave plate 54.
  • the method of isolation is not limited to this particular means and may include other known isolation means, such as a polarizer and a Faraday rotator combination.
  • the pump beam 14 is initially linearly polarized, which passes through the polarizer 52.
  • the polarization of the pump beam 14 is converted to circular polarization by the quarter wave plate 54.
  • the polarization of the backwardly propagating SBS wave 50 is also circularly polarized, but of the opposite handedness. This becomes orthogonal to that of the pump beam 14, after passing through the quarter-wave plate.
  • the SBS wave 50 is thereby reflected by the polarizer external of the apparatus 10.
  • FIG. 1 shows two focuses
  • the present invention is not so limited, and may comprise three or more focuses in a single Raman cell.
  • FIG. 3 illustrates an alternative embodiment of the present invention comprising a two cell, two focus Raman laser apparatus, generally designated as 60.
  • the same reference numerals are used for the same elements in FIG. 3 as in FIG. 1.
  • FIG. 3 differs from FIG. 1 only in that the single Raman cell 16 in FIG. 1 is replaced by two Raman cells 16' and 18, filled with Raman media 20' and 22.
  • Flat windows 21' and 23' in cell 16' and flat windows 25 and 27 is cell 18 are provided to contain the Raman medium in the respective cells.
  • Focus 34 is located in cell 16' and focus 36 is located in cell 18.
  • the lens 28 in cell 16 in FIG. 1 is replaced by lenses 28' and 30 in FIG. 3.
  • Lens 28' re-collimates the beam after propagation through cell 16'.
  • Lens 30 focuses the beam into cell 22.
  • the operation of the apparatus of FIG. 3 is the same as the operation of the apparatus of FIG. 1, previously described.
  • FIG. 3 is shown as including only the two Raman cells 16' and 18, the present invention is not so limited and, although not specifically illustrated, three or more Raman cells may be provided with a focus in each cell. Further, although the cells in FIG. 3 are illustrated as being disposed in an in-line configuration, they may be arranged in a folded or laterally offset (periscope) configuration, although not shown.
  • periscope laterally offset
  • Two and three focus backward Raman configurations in accordance with the present invention were evaluated using methane gas as the Raman medium at fill pressures from 400 to 1000 psig (28.1 to 70.3 Kg/cm2). Operating at reduced pressure lowered the Raman gain, but reduced the SBS gain by a greater amount since reduced pressure lessens competition from SBS. Fill pressures of 600 psig (42.2 Kg/cm2) were sufficient for efficient Raman conversion. Conversion efficiencies of 35% were achieved in experiments with methane.
  • Example 2 A two focus, single cell configuration was operated at 10 Hz with gas circulation at the first focus only, as discussed in Example 1.
  • the performance at 10 hertz was comparable to that at low PRF (1 Hz) operation, even though the gas was not circulated at the second focus. This demonstrates that the Raman conversion is almost entirely at the first focus.
  • the test data is presented in FIGs. 4, 5 and 6, which are discussed in Example 1.
  • a two focus, two cell configuration was tested as discussed in Example 2.
  • the test data is presented in FIG. 7, which is discussed in Example 2.
  • a three focus, single cell backward Raman configuration was also tested as discussed in Example 3, using deuterium as the Raman medium, at a pressure of 2000 psig or 140.6 Kg/cm2.
  • the vibrational Stokes shift for deuterium is 2991 cm ⁇ 1, almost the same as that of methane (2916 cm ⁇ 1), so that a laser apparatus using deuterium is also capable of producing an eye-safe output beam in response to a pump beam at 1.06 microns.
  • the purpose of these tests was to reduce SBS.
  • the SBS gain relative to SRS is low for deuterium, and little SBS was encountered in the experiments.
  • the problem with deuterium is that the Raman gain is half that of methane.
  • the three focus configuration and higher pressure partially offset the reduced gain.
  • FIG. 1 An apparatus as illustrated in FIG. 1 was constructed and tested, and consisted of a single Raman cell having two focuses.
  • the pump input energy was 163 millijoules (mJ) with a pulsewidth of 17 nanoseconds and a PRF of 10 Hz.
  • the gas cell was filled with 1000 psig (70.3 Kg/cm2) of methane.
  • the gas at the first focus (34) was circulated, but the design and construction of the apparatus did not permit circulation of the gas at the second focus (36).
  • the f/numbers of the focuses 34 and 36 were F/33 and F/20, respectively.
  • the corner cube was used, the backward Raman output was 55 mJ, and the SBS energy was 47 mJ. Without the corner cube, the backward Raman output was 40 mJ, and the SBS was 57 mJ.
  • FIG. 4 illustrates the performance of the present two focus apparatus of FIG. 1 (2-focus) as compared with a prior art Raman half resonator (designated as "1/2 resonator") with reference to the pump wave.
  • the horizontal axis represents the full divergence angle in milliradians (mrad) in the far field.
  • the vertical axis represents the fraction of total energy of the 1.54 micron beam or wave contained within the corresponding far field angle. It can be seen in FIG. 4 that a larger fraction of the total energy of the present backward SRS wave is contained in a smaller angle than for the prior art configuration. This is a measure of beam divergence, and indicates that a lower, and thereby more desirable, value of beam divergence is attainable with the present invention than with the prior art configuration.
  • FIGs. 5 and 6 are similar to FIG. 4, but depict the pump and backward SRS beam divergences of the present apparatus at low PRF (less than 1 Hz), and at 10 Hz, respectively. Although the beam divergence increases with PRF, the results are acceptable for numerous practical applications even at 10 Hz. This increase is due to turbulence caused by not circulating the gas at the second focus. Circulating the gas at the second focus should reduce the 10 Hz beam divergence.
  • FIG. 3 An apparatus as illustrated in FIG. 3 was constructed and tested, consisting of two gas cells with methane gas at 850 psig (59.8 Kg/cm2) as the Raman medium in both cells. Only the gas in the first cell was circulated.
  • the lenses 26 and 28' had focal lengths of 200 millimeters (mm), and focused the pump beam 14 into the first Raman cell 16' with an f/number of F/33.
  • the lenses 30 and 32 had focal lengths of 125 mm, and focused the pump beam 14 into the second Raman cell 18 with an f/number of F/20.
  • the energy of the input pump beam 14 was 155 mJ, at a PRF of 10 Hz.
  • the energies of the component waves in the apparatus were measured as follows: Backward SRS (output) - 52 mJ Depleted pump - 36 mJ Forward SRS between cells - 9 mJ SBS - 26 mJ
  • corner cube 42 produced the desirable result of increasing the backward SRS energy while decreasing the SBS energy.
  • FIG. 7 is a graph illustrating the performance of the apparatus of FIG. 3 (designated as "2-focus"), as compared with a single focus backward Raman configuration resulting from omitting the second Raman cell 18 (designated as "1-focus”).
  • a corner cube was used in both the 1-focus and 2-focus configurations to reflect back the forward SRS. It can be seen in FIG. 7 that the backward SRS as a function of pump beam energy is substantially greater for the present two focus configuration than for the single focus configuration.
  • FIGs. 8 and 9 illustrate the output energies of the component waves as a function of input laser pump beam energy, without and with a corner cube, respectively. It will be noted that energy of the backward SRS wave, which constitutes the output of the apparatus, was increased substantially relative to the energy of the SBS wave by adding the corner cube. In addition, the lower pressure of methane resulted in reduced SBS, while Raman conversion efficiency was good as a result of adding a third focus.

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  • Physics & Mathematics (AREA)
  • Electromagnetism (AREA)
  • Engineering & Computer Science (AREA)
  • Plasma & Fusion (AREA)
  • Optics & Photonics (AREA)
  • Lasers (AREA)
EP92102460A 1991-02-15 1992-02-14 Retro-laser Raman à foyers multiples Expired - Lifetime EP0499262B1 (fr)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
US655614 1984-09-28
US07/655,614 US5090016A (en) 1991-02-15 1991-02-15 Multiple focus backward Raman laser apparatus

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EP0499262A2 true EP0499262A2 (fr) 1992-08-19
EP0499262A3 EP0499262A3 (en) 1993-03-31
EP0499262B1 EP0499262B1 (fr) 1994-10-12

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US (1) US5090016A (fr)
EP (1) EP0499262B1 (fr)
JP (1) JP2510371B2 (fr)
KR (1) KR960000234B1 (fr)
DE (1) DE69200510T2 (fr)
ES (1) ES2061289T3 (fr)
IL (1) IL100617A (fr)
NO (1) NO303996B1 (fr)
TR (1) TR25816A (fr)

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CN105529612A (zh) * 2015-11-13 2016-04-27 华北电力大学（保定） 交叉受激散射增强装置及方法

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FR2677500B1 (fr) * 1991-06-10 1993-10-01 Cilas Laser raman.
IL102538A (en) * 1991-07-18 1995-07-31 Gec Maconi Avionics Holdings L Laser Ramen
GB9115556D0 (en) * 1991-07-18 1991-11-06 Gec Ferranti Defence Syst A raman laser
US5272717A (en) * 1992-01-17 1993-12-21 Hughes Aircraft Company Single focus backward Raman laser
US5377210A (en) * 1992-05-08 1994-12-27 The United States Of America As Represented By The Secretary Of The Air Force Self-pumped optical phase conjugation with a sodium Raman laser
KR970005166B1 (ko) * 1993-04-24 1997-04-12 국방과학연구소 유도 브릴루인 산란을 이용한 라만 레이저 발진 방법 및 그 장치
US6151337A (en) * 1998-05-06 2000-11-21 The Research And Development Institute, Inc. Continuous-wave Raman laser having a high-finesse cavity
WO2003065517A2 (fr) * 2001-11-21 2003-08-07 Science Research Laboratory, Inc. Procedes et appareils permettant de conserver la qualite d'un milieu raman dans une cellule de conversion raman
US7046432B2 (en) * 2003-02-11 2006-05-16 Coherent, Inc. Optical fiber coupling arrangement
US7583364B1 (en) * 2004-03-19 2009-09-01 University Corporation For Atmospheric Research High pulse-energy, eye-safe lidar system
US20060078016A1 (en) * 2004-10-07 2006-04-13 Dept Of The Army Matched filter used as an integral part of an SBS system for within cavity pulse reconstruction
US8452574B2 (en) 2009-02-02 2013-05-28 The United States Of America, As Represented By The Secretary Of The Navy System and method of generating atmospheric turbulence for testing adaptive optical systems
WO2010096823A1 (fr) * 2009-02-23 2010-08-26 The Goverment Of The U.S.A., As Represented By The Secretary Of The Navy Système et procédé permettant de générer des impulsions optiques très intenses sans risque pour les yeux au moyen de deux cellules raman à déplacement arrière
JP2013520804A (ja) * 2010-02-24 2013-06-06 マックォーリー・ユニバーシティ 中赤外から遠赤外のダイヤモンド・ラマンレーザーシステム及び方法
US8907260B2 (en) 2011-01-14 2014-12-09 The United States Of America, As Represented By The Secretary Of The Navy Extended source wavefront sensor through optical correlation with a change in centroid position of light corresponding to a magnitude of tip/tilt aberration of optical jitter
US9664869B2 (en) 2011-12-01 2017-05-30 Raytheon Company Method and apparatus for implementing a rectangular-core laser beam-delivery fiber that provides two orthogonal transverse bending degrees of freedom
US9535211B2 (en) 2011-12-01 2017-01-03 Raytheon Company Method and apparatus for fiber delivery of high power laser beams
US8675694B2 (en) 2012-02-16 2014-03-18 Raytheon Company Multi-media raman resonators and related system and method
US8983259B2 (en) 2012-05-04 2015-03-17 Raytheon Company Multi-function beam delivery fibers and related system and method

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Publication number Priority date Publication date Assignee Title
CN105529612A (zh) * 2015-11-13 2016-04-27 华北电力大学（保定） 交叉受激散射增强装置及方法
CN105529612B (zh) * 2015-11-13 2020-09-04 华北电力大学（保定） 交叉受激散射增强装置及方法

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EP0499262B1 (fr) 1994-10-12
TR25816A (tr) 1993-09-01
NO920490L (no) 1992-08-17
DE69200510T2 (de) 1995-02-16
JPH0582916A (ja) 1993-04-02
US5090016A (en) 1992-02-18
IL100617A (en) 1996-01-19
JP2510371B2 (ja) 1996-06-26
KR920017310A (ko) 1992-09-26
KR960000234B1 (ko) 1996-01-03
NO303996B1 (no) 1998-10-05
DE69200510D1 (de) 1994-11-17
NO920490D0 (no) 1992-02-06
EP0499262A3 (en) 1993-03-31
IL100617A0 (en) 1992-09-06
ES2061289T3 (es) 1994-12-01

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